mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

In the Linux kernel, the following vulnerability has been resolved:

mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

Initialize nr_pages to 1 at the start of each loop iteration, like

folio_referenced_one() does.

Without this, nr_pages computed by a previous folio_unmap_pte_batch() call

can be reused on a later iteration that does not run

folio_unmap_pte_batch() again.

mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call

madvise(MADV_FREE), then make the last page device-exclusive via

HMM_DMIRROR_EXCLUSIVE.

Trigger node reclaim through sysfs. Now, in try_to_unmap_one(), we will

first clear the first 15 out of 16 entries mapping the lazyfree folio.

This will set nr_pages to 15. In the next pvmw walk, this nr_pages gets

reused on a device-exclusive pte, thus potentially corrupting folio

refcount/mapcount.

At the moment, I have a userspace program which can make the kernel spit

out a trace, but the blow up is in folio_referenced_one(), because there

are existing bugs in the interaction between device-private and rmap

(which too I am investigating). I did a one liner kernel change to avoid

going into folio_referenced_one(), and the kernel blows up at

folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted.

Note that the bug is there not since file folio batching but lazyfree

folio batching, since device-exclusive only works for anonymous folios.

Userspace visible effect is simply kernel crashing somewhere due to

refcount/mapcount corruption.